There has recently been growing interest in the possibility of integrating WLAN and cellular network services. Buddhikot et al. provide a useful overview of proposed approaches to WLAN/cellular integration for wireless data services in “Integration of 802.11 and Third-Generation Wireless Data Networks,” Proceedings of IEEE Infocom 2003 (San Francisco, April, 2003), which is incorporated herein by reference.
WLANs operating in accordance with the IEEE 802.11 standards operate in the unlicensed ISM bands and support data rates between 1 Mbps and 54 Mbps, at low capital cost. WLAN access points, however, are generally limited in range, covering areas no bigger than a few thousand square meters. They are most commonly deployed indoors, in business facilities, homes and at “hot spots” in public buildings.
Cellular networks, by contrast, offer ubiquitous coverage, with much lower data rates and high capital cost. Third-generation (3G) cellular standards, such as CDMA2000 and UMTS, for example, offer peak data rates that range from 64 kbps up to 2 Mbps. Integrating WLAN and cellular services would allow network operators to expand their user base, improve their return on investment, and offer users enhanced performance. Such integration requires deployment of mobile devices with dual radio interfaces (which are already becoming available on the market) and suitable interworking between equipment and protocols on the WLAN and cellular networks.
Buddhikot et al. describe two possible approaches to the design of an integrated 3G/802.11 network, which they refer to as “tightly-coupled” and “loosely-coupled” architectures. The rationale behind the tightly-coupled approach is to make the 802.11 network appear to the 3G core network as though it were another 3G cellular access network. In this approach, the 802.11 network emulates functions that are natively available in 3G radio access networks. For this purpose, a WLAN gateway hides the details of the 802.11 network from the 3G core and implements all the 3G protocols that are required in a 3G radio access network. Mobile stations in the network are required to implement the 3G protocol stack on top of their standard 802.11 network cards, and switch between their cellular and 802.11 radio interfaces as needed. The WLAN and cellular networks would share the same authentication, signaling, transport and billing infrastructures, independently of the protocols used at the physical layer on the radio interface. The authors maintain that the tightly-coupled approach is complex, costly and inflexible, and is therefore unlikely to gain wide acceptance.
The loosely-coupled approach also uses a WLAN gateway, which in this case connects to the Internet and does not have any direct link to the elements of the 3G cellular network. Users accessing services of the WLAN gateway may include users that have locally signed on, as well as mobile users visiting from other networks. In this approach, different mechanisms and protocols can be used to handle authentication, billing and mobility management in the 3G and 802.11 portions of the network. To interwork with the 3G network, the WLAN gateway must support Mobile-IP functionalities to handle mobility across networks, and must enable the 3G provider to collect the WLAN accounting records and generate a unified billing statement, as well as applying the service policies of the 3G network in the WLAN environment. Buddhikot et al. favor the flexibility offered by the loosely-coupled approach, but they are concerned essentially with data services and do not relate to the special requirements of voice calls.
U.S. Pat. No. 6,680,923, to Leon, describes a system for hybrid communication, which provides data communication either over a computerized network, such as the Internet, or over the air, via a cellular network or other wireless network. A wireless communication device, such as a cellular telephone or personal digital assistant (PDA), has an additional transceiver that communicates with a computer using Bluetooth™ or equivalent technology, and thus can access the Internet or other computer networks. The system incorporates auto-switching, so that when the additional transceiver is not successful in achieving network access, the cellular or other wireless network is used for data communication.